// TexturedQuad.js (c) 2012 matsuda and kanda
// Vertex shader program
var VSHADER_SOURCE =
  `
    attribute vec4 a_Position;
    attribute vec2 a_TexCoord;
    varying vec2 v_TexCoord;
    void main() {
      gl_Position = a_Position;
      v_TexCoord = a_TexCoord;
    }
  `;

// Fragment shader program
var FSHADER_SOURCE =
`
  #ifdef GL_ES
  precision mediump float;
  #endif
  uniform sampler2D u_Sampler;
  varying vec2 v_TexCoord;
  void main() {
    gl_FragColor = texture2D(u_Sampler, v_TexCoord);
  }
`
function main() {
  // Retrieve <canvas> element
  var canvas = document.getElementById("webgl");

  // Get the rendering context for WebGL
  var gl = getWebGLContext(canvas);
  if (!gl) {
    console.log("Failed to get the rendering context for WebGL");
    return;
  }

  // Initialize shaders
  if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
    console.log("Failed to intialize shaders.");
    return;
  }

  let index = 0;
  setInterval(() => {
    draw(gl,  (index++ % 3)+ ".JPG");
  }, 1000);
}

function draw(gl, jpg) {
  // Set the vertex information
  var n = initVertexBuffers(gl);
  if (n < 0) {
    console.log("Failed to set the vertex information");
    return;
  }

  // Specify the color for clearing <canvas>
  gl.clearColor(0.0, 0.0, 0.0, 1.0);

  // Set texture
  if (!initTextures(gl, n, jpg)) {
    console.log("Failed to intialize the texture.");
    return;
  }
}

function initVertexBuffers(gl) {
  var verticesTexCoords = new Float32Array([
    // Vertex coordinates, texture coordinate
    -0.5,
    0.5,
    0.0,
    1.0,
    -0.5,
    -0.5,
    0.0,
    0.0,
    0.5,
    0.5,
    1.0,
    1.0,
    0.5,
    -0.5,
    1.0,
    0.0,
  ]);
  var n = 4; // The number of vertices

  // Create the buffer object
  var vertexTexCoordBuffer = gl.createBuffer();
  if (!vertexTexCoordBuffer) {
    console.log("Failed to create the buffer object");
    return -1;
  }

  // Bind the buffer object to target
  gl.bindBuffer(gl.ARRAY_BUFFER, vertexTexCoordBuffer);
  gl.bufferData(gl.ARRAY_BUFFER, verticesTexCoords, gl.STATIC_DRAW);

  var FSIZE = verticesTexCoords.BYTES_PER_ELEMENT;
  //Get the storage location of a_Position, assign and enable buffer
  var a_Position = gl.getAttribLocation(gl.program, "a_Position");
  if (a_Position < 0) {
    console.log("Failed to get the storage location of a_Position");
    return -1;
  }
  gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, FSIZE * 4, 0);
  gl.enableVertexAttribArray(a_Position); // Enable the assignment of the buffer object

  // Get the storage location of a_TexCoord
  var a_TexCoord = gl.getAttribLocation(gl.program, "a_TexCoord");
  if (a_TexCoord < 0) {
    console.log("Failed to get the storage location of a_TexCoord");
    return -1;
  }
  // Assign the buffer object to a_TexCoord variable
  gl.vertexAttribPointer(a_TexCoord, 2, gl.FLOAT, false, FSIZE * 4, FSIZE * 2);
  gl.enableVertexAttribArray(a_TexCoord); // Enable the assignment of the buffer object

  return n;
}

function initTextures(gl, n, jpg) {
  var texture = gl.createTexture(); // Create a texture object
  if (!texture) {
    console.log("Failed to create the texture object");
    return false;
  }

  // Get the storage location of u_Sampler
  var u_Sampler = gl.getUniformLocation(gl.program, "u_Sampler");
  if (!u_Sampler) {
    console.log("Failed to get the storage location of u_Sampler");
    return false;
  }
  var image = new Image(); // Create the image object
  if (!image) {
    console.log("Failed to create the image object");
    return false;
  }
  // Register the event handler to be called on loading an image
  image.onload = function () {
    loadTexture(gl, n, texture, u_Sampler, image);
  };
  // Tell the browser to load an image
  image.src = "../resources/" + jpg;

  return true;
}

function loadTexture(gl, n, texture, u_Sampler, image) {
  gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1); // Flip the image's y axis
  // Enable texture unit0
  gl.activeTexture(gl.TEXTURE0);
  // Bind the texture object to the target
  gl.bindTexture(gl.TEXTURE_2D, texture);

  // Set the texture parameters
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
  // Set the texture image
  gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, gl.RGB, gl.UNSIGNED_BYTE, image);

  // Set the texture unit 0 to the sampler
  gl.uniform1i(u_Sampler, 0);

  gl.clear(gl.COLOR_BUFFER_BIT); // Clear <canvas>

  gl.drawArrays(gl.TRIANGLE_STRIP, 0, n); // Draw the rectangle
}
